In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and image. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using ordinary paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate. For example, PCT pamphlet WO 00/39226 discloses a water-based ink containing a pigment-containing vinyl polymer in which a graft polymer obtained from a macromer is used as the vinyl polymer to achieve a high print density.
Also, in JP 10-87768A, there is disclosed a water dispersion containing an insoluble colorant and a graft copolymer dispersant having a main chain and at least one side chain in which both the chains are respectively produced from an ethylenically unsaturated monomer, and one of the main chain and side chain constitutes a hydrophilic moiety and the other constitutes a hydrophobic moiety containing at least one monomer selected from the group consisting of aryl esters of acrylic acid, aryl esters of methacrylic acid, N-aryl acrylamides, N-acryl methacrylamides and vinyl aryl esters in an amount of 50% by weight or more based on the total weight of the hydrophobic moiety.
Although these conventional water-based inks are excellent in print density and dispersion stability, there is still such a need that the water-based inks are further improved in print density upon printing on ordinary papers as well as gloss upon printing on coated papers for ink-jet printing (hereinafter referred to merely a “coated papers”).